michael berkshire administers green roof projects for the city. >> rooftops are one of the last kind of frontiers that you can really look at, and it's a significant amount of space. >> sreenivasan: berkshire says the plants on green roofs soak up heat, and keep both the buildings underneath and the air above them cooler. that, in turn, reduces the cost of air conditioning. this roof saves city hall about $3,600 a year. chicago has 359 green roofs covering almost five and a half million square feet. that's more than any other city in north america. millenium park is the largest green roof in the world at 25 acres, covering parking garages, a railway, and an opera house. >> what's interesting about a green roof is that it provides the benefits that can help mitigate in terms of, you know, keeping the temperatures lower. but then in terms of the adaptation, we're going to see more rainfall and heavier rainfall events. so the more rainfall you can absorb on-site, the less were going to overburden that combined storm sewer system. so a green roof not only helps reduce the heat, it also c